1. Field of the Invention
The present invention relates to the treatment of sewage and more particularly to the stabilization, concentration and thickening of activated sludge present in sewage treatment processes.
Conventional sewage treatment plants which employ aerobic processes, such as the activated sludge process, commonly include an aeration chamber, a clarifier, a contact chamber and a sludge holding tank. Generally, these processes involve the mixing of organic material, in the form of raw sewage, with activated sludge, which contains the requisite aerobic bacteria, and subsequently aerating the mixture in an aeration chamber until the organic material is suitably biodegraded. Thereafter, the aerated mixture of activated sludge and raw sewage, referred to as a mixed liquor, is transferred to a settling chamber or clarifier where the activated sludge solids tend to separate from the liquid effluent, which is subsequently transferred to purifying contact chambers for further treatment. Although some of the settled activated sludge may be re-cycled to the aeration chamber and mixed with raw sewage, a periodic removal of a portion of the activated sludge from the treatment system is often required. For example, the activated sludge process requires a recycling of only a portion of the settled activated sludge with the remaining portion of the settled activated sludge removed to a sludge holding tank for further treatment. Further, for example, in the extended aeration process, wherein no excess activated sludge will be produced under theoretical conditions various insoluble products tend to accumulate and it is necessary to periodically remove a portion of the settled activated sludge from the treatment system.
The settled excess activated sludge removed from the clarifiers predominantly comprises liquid matter with a relatively small amount of solids, such as organic material and bacteria. Consequently, dehydration and concentration of the excess sludge often is relatively slow and difficult. Further, if the supply of oxygen to the excess sludge is deficient, anaerobic conditions often exist which causes the sludge to become septic and undergo fermentation with the resultant production of noxious gasses and odors. Under these anaerobic conditions it is often necessary to provide covers and other expensive equipment to neutralize and contain the noxious gasses. To avoid these difficulties, it is proposed to treat the excess sludge with a supply of oxygen to stabilize the sludge from septic conditions while the sludge solids are becoming separated from the liquid component of the sludge. Accordingly, air is introduced into the excess sludge in order to circulate the sludge and provide the necessary oxygen content.
This invention also relates to a sludge thickening apparatus which is comparatively economical to fabricate and operate. More specifically, the sludge thickening apparatus of the present invention incorporates a central airlift member for circulating the sludge whereby the air supplied thereto not only causes the sludge to circulate but also provides a source of oxygen for the sludge. Further, the sludge thickening apparatus includes a central separation tank having a lower opening communicating with a sludge receiving tank whereby the liquid matter separated from the sludge solids may be drawn off from a quiescent zone in the sludge receiving tank and discharged outwardly of the treatment apparatus. Thus, the treatment apparatus of the present invention comprises a comparatively simplified and efficient sludge treatment apparatus for thickening the sludge solids while effectively separating the liquid matter therefrom without the use of costly mechanical apparatus.
2. Description of the Prior Art
Sewage treatment systems which employ air to circulate sewage are generally exemplified by U.S. Pat. Nos. 2,638,444, 3,194,756, 3,696,029 and 4,008,159. Further, treatment systems which incorporate mechanical devices and/or air to circulate and mix sewage during various stages of treatment are exemplified by U.S. Pat. Nos. 2,473,297, 2,509,695, 3,439,807, 3,152,982 and 3,595,783. Of somewhat related interest are devices exemplified by U.S. Pat. Nos. 2,314,977, 2,343,836, 2,911,102 and 3,875,066. There are many problems with one or more of the above identified prior art and this invention is designed to alleviate some of such problems.